1. Field of the Invention
This invention relates to the rotation angle detecting apparatus of a rotary encoder or the like provided with a rotary disc made chiefly of synthetic resin and the rotary disc thereof.
2. Description of Related Art
Many of conventional rotary encoders are of the optical type which optically detects rotation angle information or the magnetic type which magnetically detects rotation angle information. The rotary encoders of the optical type are of the transmitting type which transmits a beam therethrough to a scale portion for detecting a rotation angle, or the reflecting type which reflects a beam by a scale portion for detecting a rotation angle.
For example, FIG. 11 of the accompanying drawings shows a conventional rotary encoder of the transmitting type, and the motor 1 of this rotary encoder supports a rotary shaft 2 by bearings 3 and 4. A mounting hub 5 is fitted to the upper portion of the rotary shaft 2, and this mounting hub 5 is fixed to the rotary shaft 2 by a set screw 6. A rotary scale 7 which is a rotation angle information recording member is carried on the upper surface of the mounting hub 5, and the rotary scale 7 is fixed to the mounting hub 5 by a snap ring 8 fixed to the rotary shaft 2 and an adhesive agent 9. A transmitting type sensor head 11 is disposed on the upper surface of the motor 1 with an electric circuit substrate 10 interposed therebetween.
The sensor head 11 is provided with a frame 12 on the electric circuit substrate 10, and the marginal edge portion of the rotary scale 7 is disposed between the upper frame portion 12a and lower frame portion 12b of the frame 12 so as not to contact therewith. The upper frame portion 12a of the frame 12 contains therein a light emitting element 13 and a collimator lens 14 in succession from above, and the lower frame portion 12b contains therein a fixed scale 15 and a light receiving element 16 in succession from above.
The mounting hub 5 is formed of a material such as brass or aluminum excellent in machinability, and the inner diameter of a fitting hole 5a for fitting the rotary shaft 2 therein and a receiving surface 5b for carrying the rotary scale 7 thereon are machined highly accurately. The rotary scale 7 comprises a thin glass plate or a thin metal plate formed with a slit by the etching process or PET film for photoengraving.
When assembling this rotary encoder, the mounting hub 5 is fitted to the rotary shaft 2 of the motor 1, and the mounting hub 5 is positioned at a predetermined thrust level and the set screw 6 is fastened. Next, the rotary scale 7 is carried on the upper surface of the mounting hub 5, and is tentatively fixed by the snap ring 8 fixed to the rotary shaft 2. Then, the centering adjustment of the center of the recording pattern of a scale portion 7a for detecting the rotation angle of the rotary scale 7 and the center of the rotary shaft 2 is effected. Thereafter, as shown in FIG. 12 of the accompanying drawings, the rotary shaft 2 and the snap ring 8 are fixed to each other by the adhesive agent 9 and also, the rotary scale 7 and the snap ring 8 are fixed to each other by the adhesive agent 9. Lastly, the sensor head 11 is brought close to the rotary scale 7 so that the outer peripheral portion of the rotary scale 7 may be inserted between the upper frame portion 12a and the lower frame portion 12b, and is fixed to an appropriate location on the motor 1.
A divergent beam emitted from the light emitting element 13 of the sensor head 11 is transmitted through the collimator lens 14 and becomes a substantially parallel beam, and passes through the scale portion 7a for detecting the rotation angle of the rotary scale 7 and passes through the fixed scale 15 and is incident on the light receiving element 16. At this time, the sensor head 11 optically reads a change in moire fringe created with a change in the relative angular position of the scale portion 7a for detecting the rotation angle and the fixed scale 15 as a change in the quantity of light incident on the light receiving element 16, and detects the rotation angle of the rotary scale 7.
FIG. 13 of the accompanying drawings shows a conventional rotary encoder of the reflecting type, and instead of the above-described sensor head 11, a sensor head 17 for detecting the reflected beam from a scale portion 7a for detecting the rotation angle of a rotary scale 7.
To manufacture the conventional rotary encoder like this, the mounting hub 5, the set screw 6, the snap ring 8, the adhesive agent 9, etc. become necessary, and the number of portions to which the adhesive agent 9 is applied is great and further, the centering adjustment also becomes necessary, and the number of steps is great and the curtailment of manufacturing cost is difficult. Also, since the rotary scale 7 comprises a thin glass plate or a thin metal plate formed with a slit by the etching process or PET film for photoengraving, it is difficult to achieve high coaxiality accuracy and fitting accuracy.
In recent years, in the manufacture of a rotary encoder, there is known a method of curtailing manufacturing cost by integrally molding the mounting hub 5 and the rotary scale 7 from a synthetic resin material, and easily incorporating then as a rotary disc onto the rotary shaft 2. For example, the applicant discloses a rotary scale using a V-groove grating in Japanese Patent Application Laid-Open No. 60-140119, Japanese Patent Application Laid-Open No. 62-3617, Japanese Utility Model Application Laid-Open No. 5-84818, Japanese Patent Publication No. 5-39410, Japanese Patent Publication No. 5-39411, etc., and has proposed a cylinder grating type rotary scale in Japanese Patent No. 2810521, Japanese Patent No. 2862417, etc.
FIG. 14 of the accompanying drawings shows a rotary encoder provided with a rotary disc 18 integrally molded from a synthetic resin material, and this rotary disc 18 has a scale portion 18a for detecting the rotation angle thereof. The rotary disc 18 is fitted to the rotary shaft 2 of a motor 1 similar to that described above, and the rotary shaft 2 and the rotary disc 18 are secured to each other by an adhesive agent 19.
This rotary encoder does not require the mounting hub 5, the set screw 6, the snap ring 8, etc. described above. Also, the fitting accuracy of the fitting hole 18b of the rotary disc 18 to the rotary shaft 2 and the coaxiality accuracy of the pattern of the scale portion 18a for detecting the rotation angle and the fitting hole 18b of the rotary disc 18 are achieved easily. Further, the most cumbersome step of centering described above is not required. Accordingly, the number of parts is reduced and moreover, the step of centering adjustment is made unnecessary and the manufacturing cost is greatly curtailed. However, the coefficient of thermal expansion of the synthetic resin material is greater than the coefficient of thermal expansion of glass or metals and therefore, if the rotary shaft 2 is made of a metal and the rotary disc 18 is made of synthetic resin, when the environmental temperature changes greatly, the dimensions of the rotary shaft 2 and the rotary disc 18 will change in accordance with their respective coefficients of thermal expansion, and the relative position between the rotary shaft 2 and the rotary disc 18 will change. Likewise, if the sensor head 11 is formed of a synthetic resin material, the sensor head may sometimes deviate from its appropriate position in accordance with its inherent temperature characteristic factor.
Particularly, when the dimensions of the rotary disc 18 and the sensor head 11 change with respect to the thrust direction of the rotary shaft 2, the optimum spacing between the scale portion 18a for detecting the rotation angle and the sensor head 11 will change, and the output signal of the sensor head 11 will change and the detection accuracy of the sensor head 11 will be deteriorated. Also, in the worst case, the rotary disc 18 and the sensor head 11 may mechanically contact with each other to thereby cause a hindrance.
However, to solve these problems, it becomes necessary to make the mounting telerance with respect to the thrust direction when mounting the rotary disc 18 and the sensor head 11 as small as possible, and this will cause the rise of the manufacturing cost.